RFC 5147

URI Fragment Identifiers for the text/plain Media Type

Network Working Group E. Wilde
Request for Comments: 5147 UC Berkeley
Updates: 2046 M. Duerst
Category: Standards Track Aoyama Gakuin University
April 2008 URI Fragment Identifiers for the text/plain Media Type
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This memo defines URI fragment identifiers for text/plain MIME
entities. These fragment identifiers make it possible to refer to
parts of a text/plain MIME entity, either identified by character
position or range, or by line position or range. Fragment
identifiers may also contain information for integrity checks to make
them more robust.

1. Introduction
This memo updates the text/plain media type defined in RFC 2046 [3]
by defining URI fragment identifiers for text/plain MIME entities.
This makes it possible to refer to parts of a text/plain MIME entity.
Such parts can be identified by either character position or range,
or by line position or range. Integrity checking information can be
added to a fragment identifier to make it more robust, enabling
applications to detect changes of the entity.
This section gives an introduction to the general concepts of text/
plain MIME entities and URI fragment identifiers, and it discusses
the need for fragment identifiers for text/plain and deployment
issues. Section 2 discusses the principles and methods on which this
memo is based. Section 3 defines the syntax, and Section 4 discusses
processing of text/plain fragment identifiers. Section 5 shows some
examples.
1.1. What Is text/plain?
Internet Media Types (often referred to as "MIME types"), as defined
in RFC 2045 [2] and RFC 2046 [3], are used to identify different
types and sub-types of media. RFC 2046 [3] and RFC 3676 [6] specify
the text/plain media type, which is used for simple, unformatted
text. Quoting from RFC 2046 [3]: "Plain text does not provide for or
allow formatting commands, font attribute specifications, processing
instructions, interpretation directives, or content markup. Plain
text is seen simply as a linear sequence of characters, possibly
interrupted by line breaks or page breaks".
The text/plain media type does not restrict the character encoding;
any character encoding may be used. In the absence of an explicit
character encoding declaration, US-ASCII [13] is assumed as the
default character encoding. This variability of the character
encoding makes it impossible to count characters in a text/plain MIME
entity without taking the character encoding into account, because
there are many character encodings using more than one octet per
character.
The biggest advantage of text/plain MIME entities is their ease of
use and their portability among different platforms. As long as they
use popular character encodings (such as US-ASCII or UTF-8 [12]),
they can be displayed and processed on virtually every computer
system. The only remaining interoperability issue is the
representation of line endings, which is discussed in Section 4.1.

1.2. What Is a URI Fragment Identifier?
URIs are the identification mechanism for resources on the Web. The
URI syntax specified in RFC 3986 [7] optionally includes a so-called
"fragment identifier", separated by a number sign ('#'). The
fragment identifier consists of additional reference information to
be interpreted by the user agent after the retrieval action has been
successfully completed. The semantics of a fragment identifier are a
property of the data resulting from a retrieval action, regardless of
the type of URI used in the reference. Therefore, the format and
interpretation of fragment identifiers is dependent on the media type
of the retrieval result.
The most popular fragment identifier is defined for text/html
(defined in RFC 2854 [10]) and makes it possible to refer to a
specific element (identified by the value of a 'name' or 'id'
attribute) of an HTML document. This makes it possible to reference
a specific part of a Web page, rather than a Web page as a whole.
1.3. Why text/plain Fragment Identifiers?
Referring to specific parts of a resource can be very useful because
it enables users and applications to create more specific references.
Users can create references to the part they really are interested in
or want to talk about, rather than always pointing to a complete
resource. Even though it is suggested that fragment identification
methods are specified in a media type's MIME registration (see [15]),
many media types do not have fragment identification methods
associated with them.
Fragment identifiers are only useful if supported by the client,
because they are only interpreted by the client. Therefore, a new
fragment identification method will require some time to be adopted
by clients, and older clients will not support it. However, because
the URI still works even if the fragment identifier is not supported
(the resource is retrieved, but the fragment identifier is not
interpreted), rapid adoption is not highly critical to ensure the
success of a new fragment identification method.
Fragment identifiers for text/plain, as defined in this memo, make it
possible to refer to specific parts of a text/plain MIME entity,
using concepts of positions and ranges, which may be applied to
characters and lines. Thus, text/plain fragment identifiers enable
users to exchange information more specifically, thereby reducing the
time and effort that is necessary to manually search for the relevant
part of a text/plain MIME entity.

The text/plain format does not support the embedding of links, so in
most environments, text/plain resources can only serve as targets for
links, and not as sources. However, when combining the text/plain
fragment identifiers specified in this memo with out-of-line linking
mechanisms such as XLink [14], it becomes possible to "bind" link
resources to text/plain resources and thereby "embed" links into
text/plain resources. Thus, the text/plain fragment identifiers
specified in this memo open a path for text/plain files to become
bidirectionally navigable resources in hypermedia systems such as the
Web.
1.4. Incremental Deployment
As long as text/plain fragment identifiers are not supported
universally, it is important to consider the implications of
incremental deployment. Clients (for example, Web browsers) not
supporting the text/plain fragment identifier described in this memo
will work with URI references to text/plain MIME entities, but they
will fail to locate the sub-resource identified by the fragment
identifier. This is a reasonable fallback behavior, and in general,
users should take into account the possibility that a program
interpreting a given URI will fail to interpret the fragment
identifier part. Since fragment identifier evaluation is local to
the client (and happens after retrieving the MIME entity), there is
no reliable way for a server to determine whether a requesting client
is using a URI containing a fragment identifier.
1.5. Notation Used in This Memo
The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [4].
2. Fragment Identification Methods
The identification of fragments of text/plain MIME entities can be
based on different foundations. Since it is not possible to insert
explicit, invisible identifiers into a text/plain MIME entity (for
example, as used in HTML documents, implemented through dedicated
attributes), fragment identification has to rely on certain inherent
properties of the MIME entity. This memo specifies fragment
identification using four different methods, which are character
positions and ranges, and line positions and ranges, augmented by an
integrity check mechanism for improving the robustness of fragment
identifiers.

When interpreting character or line numbers, implementations MUST
take the character encoding of the MIME entity into account, because
character count and octet count may differ for the character encoding
being used. For example, a MIME entity using the UTF-16 encoding (as
specified in RFC 2781 [11]) uses two octets per character in most
cases, and sometimes four octets per character. It can also have a
leading BOM (Byte-Order Mark), which does not count as a character
and thus also affects the mapping from a simple octet count to a
character count.
2.1. Fragment Identification Principles
Fragment identification can be done by combining two orthogonal
principles, which are positions and ranges, and characters and lines.
This section describes the principles themselves, while Section 2.2
describes the combination of the principles.
2.1.1. Positions and Ranges
A position does not identify an actual fragment of the MIME entity,
but a position inside the MIME entity, which can be regarded as a
fragment of length zero. The use case for positions is to provide
pointers for applications that may use them to implement
functionalities such as "insert some text here", which needs a
position rather than a fragment. Positions are counted from zero;
position zero being before the first character or line of a text/
plain MIME entity. Thus, a text/plain MIME entity having one
character has two positions, one before the first character (position
zero), and one after the first character (position 1).
Since positions are fragments of length zero, applications SHOULD use
other methods than highlighting to indicate positions, the most
obvious way being the positioning of a cursor (if the application
supports the concept of a cursor).
Ranges, on the other hand, identify fragments of a MIME entity that
have a length that may be greater than zero. As a general principle
for ranges, they specify both a lower and an upper bound. The start
or the end of a range specification may be omitted, defaulting to the
first or last position of the MIME entity, respectively. The end of
a range must have a value greater than or equal to the start. A
range with identical start and end is legal and identifies a range of
length zero, which is equivalent to a position.
Applications that support a concept such as highlighting SHOULD use
such a concept to indicate fragments of lengths greater than zero to
the user.

For positions and ranges, it is implicitly assumed that if a number
is greater than the actual number of elements in the MIME entity,
then it is referring to the last element of the MIME entity (see
Section 4 for details).
2.1.2. Characters and Lines
The concept of positions and ranges can be applied to characters or
lines. In both cases, positions indicate points between these
entities, while ranges identify zero or more of these entities by
indicating positions.
Character positions are numbered starting with zero (ignoring initial
BOM marks or similar concepts that are not part of the actual textual
content of a text/plain MIME entity), and counting each character
separately, with the exception of line endings, which are always
counted as one character (see Section 4.1 for details).
Line positions are numbered starting with zero (with line position
zero always being identical with character position zero);
Section 4.1 describes how line endings are identified. Fragments
identified by lines include the line endings, so applications
identifying line-based fragments MUST include the line endings in the
fragment identification they are using (e.g., the highlighted
selection). If a MIME entity does not contain any line endings, then
it consists of a single (the first) line.
2.2. Combining the Principles
In the following sections, the principles described in the preceding
section (positions/ranges and characters/lines) are combined,
resulting in four use cases. The schemes mentioned below refer to
the fragment identifier syntax, described in detail in Section 3.
2.2.1. Character Position
To identify a character position (i.e., a fragment of length zero
between two characters), the 'char' scheme followed by a single
number is used. This method identifies a position between two
characters (or before the first or after the last character), rather
than identifying a fragment consisting of a number of characters.
Character position counting starts with zero, so the character
position before the first character of a text/plain MIME entity has
the character position zero, and a MIME entity containing n distinct
characters has n+1 distinct character positions, the last one having
the character position n.

2.2.2. Character Range
To identify a fragment of one or more characters (a character range),
the 'char' scheme followed by a range specification is used. A
character range is a consecutive region of the MIME entity that
extends from the starting character position of the range to the
ending character position of the range.
2.2.3. Line Position
To identify a line position (i.e., a fragment of length zero between
two lines), the 'line' scheme followed by a single number is used.
This method identifies a position between two lines (or before the
first or after the last line), rather than identifying a fragment
consisting of a number of lines. Line position counting starts with
zero, so the line position before the first line of a text/plain MIME
entity has the line position zero, and a MIME entity containing n
distinct lines has n+1 distinct line positions, the last one having
the line position n.
2.2.4. Line Range
To identify a fragment of one or more lines (a line range), the
'line' scheme followed by a range specification is used. A line
range is a consecutive region of the MIME entity that extends from
the starting line position of the range to the ending line position
of the range.
2.3. Fragment Identifier Robustness
It is easily possible that a modification of the referenced resource
will break a fragment identifier. If applications want to create
more robust fragment identifiers, they may do so by adding integrity-
check information to fragment identifiers. Such information is used
to detect changes in the resource. Applications can then warn users
about the possibility that a fragment identifier might have been
broken by a modification of the resource.
Fragment identifiers are interpreted by clients, and therefore
integrity-check information is defined on MIME entities rather than
on the resource itself. This means that the integrity-check
information is specific to a certain entity. Specifically, content
encodings and/or content transfer encodings must be removed before
using integrity-check information.
Integrity-check information may specify the character encoding that
has been used when creating the information, and if such a
specification is present, clients MUST check whether the character

encoding specified and the character encoding of the retrieved MIME
entity are equal, and clients MUST NOT use the integrity check
information if these values differ. However, clients MAY choose to
transcode the retrieved MIME entity in the case of differing
character encodings, and after doing so, apply integrity checks.
Please note that this method is inherently unreliable because certain
characters or character sequences may have been lost or normalized
due to restrictions in one of the character encodings used.
3. Fragment Identification Syntax
The syntax for the text/plain fragment identifiers is
straightforward. The syntax defines four schemes, 'char', 'line',
and integrity check (which can either be 'length' or 'md5'). The
'char' and 'line' schemes can be used in two different variants,
either the position variant (with a single number), or the range
variant (with two comma-separated numbers). An integrity check can
either use the 'length' or the 'md5' scheme to specify a value.
'length' in this case serves as a very weak but easy to calculate
integrity check.
The following syntax definition uses ABNF as defined in RFC 5234 [9],
including the rules DIGIT and HEXDIG. The mime-charset rule is
defined in RFC 2978 [5].
NOTE: In the descriptions that follow, specified text values MUST be
used exactly as given, using exactly the indicated lower-case
letters. In this respect, the ABNF usage differs from [9].
text-fragment = text-scheme 0*( ";" integrity-check )
text-scheme = ( char-scheme / line-scheme )
char-scheme = "char=" ( position / range )
line-scheme = "line=" ( position / range )
integrity-check = ( length-scheme / md5-scheme )
[ "," mime-charset ]
position = number
range = ( position "," [ position ] ) / ( "," position )
number = 1*( DIGIT )
length-scheme = "length=" number
md5-scheme = "md5=" md5-value
md5-value = 32HEXDIG
3.1. Integrity Checks
An integrity check can either specify a MIME entity's length, or its
MD5 fingerprint. In both cases, it can optionally specify the
character encoding that has been used when calculating the integrity

check, so that clients interpreting the fragment identifier may check
whether they are using the same character encoding for their
calculations. For lengths, the character encoding can be necessary
because it can influence the character count. As an example, Unicode
includes precomposed characters for writing Vietnamese, but in the
windows-1258 encoding, also used for writing Vietnamese, some
characters have to be encoded with separate diacritics, which means
that two characters will be counted. Applying Unicode terminology,
this means that the length of a text/plain MIME entity is computed
based on its "code points". For MD5 fingerprints, the character
encoding is necessary because the MD5 algorithm works on the binary
representation of the text/plain resource.
To allow future changes to this specification to address developments
in cryptography, implementations MUST ignore new types of integrity
checks, with names other than 'length' and 'md5'. If several
integrity checks are present, an application can use whatever
integrity checks it understands, and among these, those integrity
checks that provide an appropriate trade-off between performance and
the need for integrity checking. Please see Section 4.3 for further
details.
The length of a text/plain MIME entity is calculated by using the
principles defined in Section 2.1.2. The MD5 fingerprint of a text/
plain MIME entity is calculated by using the algorithm presented in
[1], encoding the result in 32 hexadecimal digits (using uppercase or
lowercase letters) as a representation of the 128 bits that are the
result of the MD5 algorithm. Calculation of integrity checks is done
after stripping any potential content-encodings or content-transfer-
encodings of the transport mechanism.
4. Fragment Identifier Processing
Applications implementing support for the mechanism described in this
memo MUST behave as described in the following sections.
4.1. Handling of Line Endings in text/plain MIME Entities
In Internet messages, line endings in text/plain MIME entities are
represented by CR+LF character sequences (see RFC 2046 [3] and RFC
3676 [6]). However, some protocols (such as HTTP) additionally allow
other conventions for line endings. Also, some operating systems
store text/plain entities locally with different line endings (in
most cases, Unix uses LF, MacOS traditionally uses CR, and Windows
uses CR+LF).
Independent of the number of bytes or characters used to represent a
line ending, each line ending MUST be counted as one single

character. Implementations interpreting text/plain fragment
identifiers MUST take into account the line ending conventions of the
protocols and other contexts that they work in.
As an example, an implementation working in the context of a Web
browser supporting http: URIs has to support the various line ending
conventions permitted by HTTP. As another example, an implementation
used on local files (e.g., with the file: URI scheme) has to support
the conventions used for local storage. All implementations SHOULD
support the Internet-wide CR+LF line ending convention, and MAY
support additional conventions not related to the protocols or
systems they work with.
Implementers should be aware of the fact that line endings in plain
text entities can be represented by other characters or character
sequences than CR+LF. Besides the abovementioned CR and LF, there
are also NEL and CR+NEL. In general, the encoding of line endings
can also depend on the character encoding of the MIME entity, and
implementations have to take this into account where necessary.
4.2. Handling of Position Values
If any position value (as a position or as part of a range) is
greater than the length of the actual MIME entity, then it identifies
the last character position or line position of the MIME entity. If
the first position value in a range is not present, then the range
extends from the start of the MIME entity. If the second position
value in a range is not present, then the range extends to the end of
the MIME entity. If a range scheme's positions are not properly
ordered (i.e., the first number is less than the second), then the
fragment identifier MUST be ignored.
4.3. Handling of Integrity Checks
Clients are not required to implement the handling of integrity
checks, so they MAY choose to ignore integrity check information
altogether. However, if they do implement integrity checking, the
following applies:
If a fragment identifier contains one or more integrity checks, and a
client retrieves a MIME entity and, using some integrity check(s),
detects that the entity has changed (observing the character encoding
specification as described in Section 3.1, if present), then the
client SHOULD NOT interpret the text/plain fragment identifier. A
client MAY signal this situation to the user.

4.4. Syntax Errors in Fragment Identifiers
If a fragment identifier contains a syntax error (i.e., does not
conform to the syntax specified in Section 3), then it MUST be
ignored by clients. Clients MUST NOT make any attempt to correct or
guess fragment identifiers. Syntax errors MAY be reported by
clients.
5. Examples
The following examples show some usages for the fragment identifiers
defined in this memo.
http://example.com/text.txt#char=100
This URI identifies the position after the 100th character of the
text.txt MIME entity. It should be noted that it is not clear which
octet(s) of the MIME entity this will be without retrieving the MIME
entity and thus knowing which character encoding it is using (in case
of HTTP, this information will be given in the Content-Type header of
the response). If the MIME entity has fewer than 100 characters, the
URI identifies the position after the MIME entity's last character.
http://example.com/text.txt#line=10,20
This URI identifies lines 11 to 20 of the text.txt MIME entity. If
the MIME entity has fewer than 11 lines, it identifies the position
after the last line. If the MIME entity has less than 20 but at
least 11 lines, it identifies the range from line 11 to the last line
of the MIME entity.
https://example.com/text.txt#line=,1
This URI identifies the first line. Please note that the URI scheme
has been changed to https.
ftp://example.com/text.txt#line=10,20;length=9876,UTF-8
As in the second example, this URI identifies lines 11 to 20 of the
text.txt MIME entity. The additional length integrity check
specifies that the MIME entity has a length of 9876 characters when
encoded in UTF-8. If the client supports the length scheme, it may
test the retrieved MIME entity for its length, but only if the
retrieved MIME entity uses the UTF-8 encoding or has been locally
transcoded into this encoding.

Please note that the FTP protocol, as well as some other protocols
underlying some other URI schemes, do not provide explicit
information about the media type of the resource being retrieved.
Using fragment identifiers with such URI schemes is therefore
inherently unreliable. Current user agents use various heuristics to
infer some media type for further processing. Processing of the
fragment identifier according to this memo is only appropriate if the
inferred media type is text/plain.
6. IANA Considerations
IANA has added a reference to this specification in the text/plain
Media Type registration.
7. Security Considerations
The fact that software implementing fragment identifiers for plain
text and software not implementing them differs in behavior, and the
fact that different software may show documents or fragments to users
in different ways, can lead to misunderstandings on the part of
users. Such misunderstandings might be exploited in a way similar to
spoofing or phishing.
In particular, care has to be taken if fragment identifiers are used
together with a mechanism that allows showing only the part of a
document identified by a fragment. One scenario may be the use of a
fragment identifier to hide small-print legal text. Another scenario
may be the inclusion of site-key-like material, which may give the
user the impression of using the real site rather than a fake site;
other scenarios may also be possible. Possible countermeasures may
include but are not limited to displaying the included content within
clearly visible boundaries and limiting inclusion to material from
the same security realm or from realms that give explicit permission
to be included in another realm.
Please note that the above issues all apply to the client side;
fragment identifiers are not used when resolving a URI to retrieve
the representation of a resource, but are only applied on the client
side.
Implementers and users of fragment identifiers for plain text should
also be aware of the security considerations in RFC 3986 [7] and RFC
3987 [8].

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